Use of rare earth elements for reducing nozzle deposits in the continuous casting of steel process

ABSTRACT

A CONTINUOUS CASTING PROCESS WHEREIN RARE EARTH METALS OR RARE EARTH SILICIDES OR OTHER ALLOYS OF THE RARE EARTH METALS ARE USED TO DEOXIDIZE STEEL TO THEREBY PREVENT THE CLOGGING OF TUNDISH METERING NOZZLES NAD PERMIT THE UNINTERRUPTED CONTINUOUS CASTING OF LARGE PRODUCTION STEEL HEATS.

United States Patent 3,623,862 USE OF RARE EARTH ELEMENTS FOR REDUCING NOZZLE DEPOSITS IN THE CONTINUOUS CAST- ING OF STEEL PROCESS Arthur F. Spengler, .lr., Park Forest, and William P. Young, La Grange, Ill., assignors to International Harvester Company, Chicago, Ill. N0 Drawing. Filed June 24, 1968, Ser. No. 739,157 Int. Cl. C21c 7/06; B22d 11/00 US. Cl. 75-57 20 Claims ABSTRACT OF THE DISCLOSURE A continuous casting process wherein rare earth metals or rare earth silicides or other alloys of the rare earth metals are used to deoxidize steel to thereby prevent the clogging of tundish metering nozzles and permit the uninterrupted continuous casting of large production steel heats.

This invention relates to a method of continuously casting molten metals.

In the continuous casting process molten steel is poured from a ladle into a tundish containing one or more nozzles. These nozzles are made from zirconia, alumina or other highly refractory materials through which metal flows and is metered into the molds forming continuous cast billets. Difficulties have always been encountered in keeping the metering nozzles open so that a continuous even flow of molten metal can be fed into the billet molds. When steel is deoxidized with aluminum, titanium, zirconium or calcium, the nozzles tend to clog, finally becoming completely closed. This terminates the casting operation. As a result, the rest of the heat must be cast in ingots. These ingots must be rolled into billets thereby adding substantially to the cost of the finished hot rolled bars. The clogging of the nozzles results primarily from the precipitation of refractory oxides from the molten metal onto the surface of the metering nozzles. This forms a web or skeleton which becomes entwined with wire-like stringers of solidified steel. As the molten metal flows through the tundish nozzle, this structure grows outward from the surface of the nozzle orifice until the flow of metal into billet molds is cut off. Then the cast must be terminated.

Deoxidation of steels with aluminum, calcium, titanium and zirconium reduces the surface tension of molten metal and tends to promote the formation of dross. These materials have been used for deoxidizing steels produced by the Bessemer converter, open hearth and electric furnace processes for many years and more recently by the basic oxygen furnace. Problems have not been encountered in the production of ingot steel because the steel flows into the ingot mold through a relatively large opening in the bottom-pour ladles which are used. Prior to the advent of the electric furnace, aluminum killed steel could not be used for lack of fluidity in producing steel casting by the open hearth and Bessemer processes. This problem was overcome to a large degree with the use of higher pouring temperatures which became possible by using the electric furnace.

During the initial development of the continuous casting process for steel, no serious problems developed due to the use of aluminum killed steels. This was primarily due to the small heats cast and the relatively high temperatures made possible by using electric melting. However, when basic oxygen furnaces came to be used in conjunction with the continuous casting process, a number of serious problems developed. First, due to the high oxygen content of basic oxygen furnace processed steels, increased amounts of aluminum and other deoxidizers were required thus causing a marked decline in metal 3,623,862 Patented Nov. 30, 1971 fluidity. .Second, due to increases in the size of the heats cast, additional time was required to cast these larger heats resulting in greater losses in metal temperature. Third, the plugging of tundish nozzles was the most serious problem to occur. It was found that large basic oxygen furnace heats, of 140 tons, for example, deoxidized with aluminum, calcium, titanium or zirconium in the elemental forms or as ferroalloys when cast at temperatures of aproximately 3000 F. form a skeleton-like deposit in tundish nozzles causing a gradual restriction of metal fiow through the tundish nozzles. Eventually the cast must be terminated. The problem is most pronounced when zirconia or high alumina tundish nozzles are used.

The problem of the plugging of tundish nozzles is serious, and prevents the successful utilization of the continuous casting process On a large scale by the steel industry. Examination of plugged tundish nozzles reveals that the skeleton-like deposit which restricts the metal flow consists primarily of alumina A1 0 along with other complex refractory materials containing oxides of aluminum, calcium, and silicon. Further studies using special test heats of steel deoxided with aluminum, calcium silicon alloys and rare earths show that the refractory materials blocking tundish nozzles are products of aluminum and calcium deoxidizing agents.

The efforts that have been made to overcome the tundish nozzle clogging problem thus far have not been too satisfactory. One method that has been tried, for example, is that of introducing aluminum wire as a deoxidizer into the molten steel after the steel has been metered through the tundish nozzle. This has been unsatisfactory because it is difficult to get a uniform mixing of the aluminum in the steel at that late stage. Furthermore, in using that method an excessive amount of aluminum oxide slag forms in the mold which leads to breakouts and causes surface defects on the billets referred to as slag pockets.

Stoppered nozzles have also been used in the tundish similar to the stopper in a steel ladle to teem ingots. This permits the use of graphite nozzles and fire clay nozzles with a larger diameter hole. These types of nozzles, however, erode away and become larger as the steel is cast. With this arrangement, the rate of flow through the nozzle was controlled by an operator. During casting times of one hour or more it is quite difficult to properly control the casting speed. Stoppered nozzles are not a satisfactory answer to the problem especially when casting large heats of tons and over.

Thus, a principal object of the present invention is to eliminate tundish nozzle clogging to permit a free flow of molten steel during a continuous casting process when large heats are being poured.

Another object of this invention is to provide an improved method of deoxidizing a heat of steel whereby the oxide precipitation problem is reduced or eliminated so that the continuous casting of the steel heat may proceed without interruption.

A further object of this invention is to utilize rare earth metals or rare earth silicides or other alloys of the rare earth metals to deoxidize steel during a continuous casting process so that precipitation of refractory oxides from the molten metal onto the surface of the tundish metering nozzles does not occur.

A still further object is to provide an improved method of continuous casting of steel which can be extended to a greater variety of steels than heretofore has been possible.

In using the basic oxygen process to prepare a large heat steel, -200 tons, for example, to be continuous cast, tundish nozzles made of zirconia have been used. Zirconia or other highly refractory material must be used to withstand not only the high heat but also the long duration of high temperature of the large heat of steel, which must pass through these nozzle structures.

In a typical example of equipment used in the continuous casting process, a tundish is used which holds about 4000 pounds of steel. Eight nozzles of /8 in diameter are connected to the tundish and meter the molten steel from the tundish into molds to form continuous cast billets of 5" or 6" squares. It takes approximately one hour to pour a 140 ton heat of steel through these eight nozzles. The importance of keeping these nozzles completely open can be readily appreciated from these facts. On the other hand, it is equally important that the nozzle does not erode and increase in size to cause an undesirable pouring rate. When aluminum was used as a deoxidizing agent, we found by X-ray diffraction techniques that the deposit which formed in the tundish nozzles exhibited a diffraction pattern identical with that of aluminum calcium oxide and aluminum oxide.

In directing our efforts to the problem of eliminating the clogging of the tundish nozzles we learned that when a rare earth metal, yttrium metal, or a compound of a rare earth or yttrium other than an oxide, and particularly a silicide was substituted for the normally used aluminum as the deoxidizing agent the undesirable clogging of the nozzles did not occur.

We have found, for example, that the use of 250-350 pounds of any rare earth silicide or yttrium silicide or mixtures of such silicides to deoxidize a 140 ton heat of steel to be continuous cast has provided adequate deoxidation. No clogging of the zirconia tundish nozzles occurred when the rare earth silicide was used. We have found that mixtures of rare earth silicides of the following composition have achieved satisfactory results.

Example 1, Example 2,

Elements percent percent Neodymium 2. 0 5. 0 Yttrium group 0.1 1.0 Iron. 40. 4 0. 4

Rare earth metal or mixtures of rare earth metals such as mischmetal have also been used satisfactorily as the deoxidizing agent in place of aluminum. An analysis of one mixture of rare earth metals used satisfactorily is as follows:

Rare earth silicides of different compositions containing from about 10%50% rare earth metals are commercially avaliable and are satisfactory for use as a deoxidizing agent in this process. The amount of rare earth silicide needed will decrease as the amount of such an element as praseodymium present in the composition increases. Praseodymium, for example, is one of the most active of the rare earth metals and is effective to release a large amount of free energy during a deoxidizing reaction.

We have found that steels tapped out of a furnace with no prior deoxidation can be treated in the ladle with the following sequence of additions and successfully continuously cast through a tundish with metering nozzles. Five pounds of silicon metal per ton are added after about five percent of the steel is tapped into the ladle. The silicon addition is then followed with an appropriate manganese addition. If other alloys are to be added they may be added With the manganese. The rare earth metals preferably are added when approximately sixty-five percent of the steel has been tapped into the ladle to allow adequate mixing. These may be added as a pure mixture of metals, referred to as mischmetal, or in the form of a silicon alloy. Steel made in this manner can be successfully cast with a rare earth metal addition of one-half pound per ton in the medium and high carbon range considering medium range to include steels having about 0.30% to 0.50% carbon and high range about 0.50% to 0.80% carbon. As the steel carbon content decreases, an increase in the rare earth addition is required. In steel having about 0.15% carbon content approximately one pound of rare earth metal per ton of steel is required.

We have also found that by using a rare earth metal composition or rare earth silicide as described above we are able to continuously cast large heats of steel in the low carbon range, something which heretofore has been impossible. Using the original calcium silicide deoxidation practice the grades of steel which could be cast were limited to medium and high carbon steels containing a minimum of 0.25% silicon. Using rare earth silicide deoxidation practice, low carbon steels with 0.15 maximum silicon are now being continuous cast. Using the rare earth silicide has also made possible the continuous casting of low carbon re-blown heats.

Thus it will be apparent that numerous advantages flow from the use of rare earth metals or rare earth silicide in place of aluminum to deoxidize steel being poured by the continuous casting process. First of all, it results in the elimination of tundish nozzle blockage. Since the tundish nozzles then last longer repair time and expense is saved. The restrictions on the grades of steel which can be continuous cast are also eliminated.

Previously, the grades of steel which could be continuous cast were limited to medium and high carbon steels containing a minimum of 0.25% silicon. Now low carbon steels with a maximum of 0.15 silicon can be continuous cast using rare earth silicide deoxidation practice.

Most so-called constructional and A151 steels can now be continuous cast using this improved process. Furthermore, the formation of slag in the copper billet molds is significantly reduced thus reducing the possibility of break outs as the billet is extracted from the bottom of the mold, Since complete heats of tons and more can now be continuous cast without clogging of the tundish nozzles production costs are slashed. Previously, an unfinished heat to be continuous cast had to be salvaged by pouring the metal into large ingots. These then had to be rolled to a smaller billet size. Thus, what previously had been the source of such excessive production costs, to the point where continuous casting of large heats was almost non-feasible from an economic standpoint, has been eliminated. Continuous casting of the large heats is now possible and is being carried out in production.

Using the rare earth metal or rare earth silicide as a deoxidizing agent also is effective to increase the pouring speed during the continuous casting process. Although in the above examples rare earth metals and rare earth silicides were used as deoxidizing agents, it is to be understood that rare earth and yttrium carbides, chlorides and other rare earth and yttrium compounds other than the oxides, such compounds being reactive with oxygen, could be used in lieu of such rare earth and yttrium metals and silicides in new process without departing from the spirit and scope of the invention.

Likewise, scandium metal, scandium compounds other than the oxide, particularly scandium silicide, and metals and compounds other than the oxides of the actinide series, such as thorium, uranium, and actinuim metals and silicides are also suitable and anti clogging agents but on the basis of availability and costs as Well as all around efliciency and results, yttrium and the lanthanide series of rare earths are preferred. While the use of aluminum as a deoxidizing agent causes an increase in surface tension of the molten steel or a decrease in fluidity, the use of rare earth metals or rare earth silicide on the other hand reduces the surface tension and consequently increases the fluidity.

It will be understood that various changes and modifications may be made which provide the characteristics of this invention without departing from the spirit thereof, and the invention is to be given its fullest possible interpretation Within the terms of the following claims.

We claim:

1. The process of preventing tundish nozzle clogging during the continuous casting of steel comprising the step of adding amount of a deoxidizing agent selected from the group consisting of rare earth metals, yttrium metal, rare earth compounds other than the oxides, yttrium compounds other than the oxides, and mixtures thereof to the molten steel before the steel is poured through the tundish nozzle, on the basis of the rare earth metal content of the deoxidizing agent, the amount of deoxidizing agent added being at least one-half pound of metal per ton of steel,

2. The process of claim 1 in which the deoxidizing agent is a rare earth metal.

3. The process of claim 1 in which the deoxidizing agent is a rare earth silicide.

4. The process of claim 1 in which the deoxidizing agents is praseodymium metal, of praseodymium silicide.

5. The process of claim 1 in which the deoxidizing agent is mischmetal.

6. The process of claim 1 in which the deoxidizing agent is a deoxidizing composition including praseodymrum.

7. The process of claim 1 in which the deoxidizing agent is a mixture having the following approximate composition in percent by weight:

Cerium 47 Lanthanum 25 Praseodymium 6 Neodymium 19 Samarium 2 Gadolinium Yttrium and other rare earth metals .5

8. The process of claim 1 in which the deoxidizing agent is a rare earth silicide mixture of the following approximate composition on the basis of the metal in percent by weight:

Silicon 2830 Lanthanum 1533 Cerium 13-29 Praseodymium 2 Neodymium 2-5 Yttrium group .1-1 Iron -40 9. The process of claim 2 wherein the rare earth metal is added to the steel in the proportion of one-half to two and one-half pound of rare earth metal per ton of steel.

10. The process of claim 13 wherein the rare earth silicide is added to the steel in the proportion of one to twenty-five pounds of silicide per ton of steel.

11. The process of continuous casting of steel comprising the steps of adding an amount of a rare earth metal or a rare earth compound reactive with oxygen to the molten steel and then pouring the steel through tundish nozzles made of a high refractory material, on the basis of the rare earth metal content of the rare earth metal or rare earth compound added, the amount of rare earth metal or rare earth compound added being at least one-half pound of metal per ton of steel.

12. The process of claim 11 wherein said amount of rare earth metal or rare earth compound added being about one-half to two and one-half pounds of metal per ton of steel.

13. The process of continuous casting of steel comprising the steps of adding an amount of a rare earth metal silicide to the molten steel and then pouring the steel through tundish nozzles made of a high refractory material, on the basis of the rare earth metal content of the rare earth silicide added, the amount added being at least one-half pound of metal per ton of steel.

14. The process of claim 13 wherein at least about one pound of rare earth silicide per ton of steel is added to the molten steel.

15. The process of claim 13 wherein about one to twenty-five pounds of rare earth silicide per ton of steel is added to the molten steel.

16. The process of continuous casting of steel comprising the steps of adding an amount of a composition consisting essentially of a rare earth metal or a rare earth compound other than oxide to the molten steel to deoxidize the steel and then pouring the steel through tundish nozzles made of a high refractory material, on the basis of the rare earth metal content of the composition added, the amount of composition added being at least one-half pound of metal per ton of steel.

17. The process of claim 11 wherein the tundish nozzle refractory material is from the group consisting of zirconia and alumina.

18. The process of claim 11 wherein the steel to be cast has a maximum of 0.15% silicon content.

19. The process of claim 16 wherein said added composition is a rare earth silicide and the steel to be cast has a maximum of 0.15% silicon content.

20. The process of preventing tundish nozzle clogging during the. continuous casting of steel comprising the step of adding an amount of a deoxidizing agent selected from the group consisting of scandium metal, scandium silicide, and actinide metal, and an actinide silicide to molten steel before the steel is poured through the tundish nozzle, on the basis of rare earth metal content of the deoxidizing agent, the amount of deoxidizing agent added being at least one-half pound of metal per ton of steel.

References Cited UNITED STATES PATENTS 2,861,908 11/1958 Mickelson -58 X 3,185,652 5/1965 Kleber 75-152 X 3,189,956 6/1965 Longden 75-58 X 3,295,963 1/1967 Galvin 75-123 X 3,467,167 9/1969 Mahin 75--57 X L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner US. Cl. X.R. 

